According to a recent trend toward downsizing of various devices, there have been strong demands for high integration of semiconductor ICs and hence resist patterns have been required to be made more and more minute so as to meet the demands. In order to satisfy this requirement, it is necessary to adopt a photolithographic process including exposure to light in a shorter wavelength range. The light for exposure has thus become shorter and shorter in wavelength, and has changed from visible light to UV or far UV light. Further, the exposure is now often carried out by use of extreme UV light. For example, in manufacture of semiconductor devices such as ICs and LSIs, specifically, in production processes of DRAMs, flash memories and logic semiconductor devices, it is required to form ultrafine patterns and hence lithography with extreme UV light is becoming more and more important.
To cope with the above requirement, researchers have developed many resist compositions sensitive to light in various wavelength ranges. Among them, commercially available chemically amplified resist compositions have been hitherto thought to be mostly employable in a photolithographic process with extreme UV light. For example, common resist compositions for KF or ArF laser exposure have been regarded as also usable in a lithographic process including exposure to extreme UV light. Actually, however, there remain many problems needed to improve in view of resolution, sensitivity, roughness, and so on.
On the other hand, there also remain problems of light sources and masks in exposure apparatuses, and those problems are reasons why lithography technique with extreme UV light has not yet been widely employed in practice. It has been thought that resist pattern shapes are impaired by longer wavelength light, particularly, deep UV light of, for example, 193 nm or 248 nm, contained in emission from extreme UV light sources. When resist compositions for KF or ArF laser exposure are used as described above in a lithographic process with extreme UV light, they are sensitively made to react not only by extreme UV light but also, of course, by deep UV light in a longer wavelength range than extreme UV light.
Extreme UV light sources generally emit extreme UV light together with longer wavelength light, such as deep UV light. Accordingly, when a fine pattern is intended to be formed according to a lithographic process by use of extreme UV light, it is preferred to adopt a light source less emitting the longer wavelength light. In order to remove deep UV light from radiation emitted by exposure apparatuses, the way of generating extreme UV light is commonly controlled. For example, optical systems in the apparatuses are regulated so as to remove deep UV light from emission of light sources. It is, however, very difficult to remove deep UV light completely from emission of conventional light sources, and hence it has been impossible in conventional exposure apparatuses to reduce the ratio of deep UV light down to 3% or less in exposure light. Since deep UV light thus contained in emission from extreme UV light sources causes impairment of resist patterns in view of roughness and pattern shape, it has been desired to improve this problem.
Meanwhile, the exposure to extreme UV light is generally carried out under high vacuum conditions. Accordingly, when a resist layer is subjected to the exposure in a lithographic process, it often gives off gases of volatile substances, which are low molecular weight compounds formed by photochemical reactions and/or components of the resist composition contained in the resist layer, such as, photosensitive materials and photo acid-generating agents. Those gases are referred to as “outgases”, and they often stain photo-masks and/or optical elements such as mirrors in exposure apparatuses to degrade the exposure accuracy. Accordingly, it has been also desired to prevent the resist layer from releasing gases.
As a means for solving the above problems, a method is developed in which the top surface of a resist layer is coated with a topcoat layer which transmits extreme UV light but absorbs deep UV light and also which prevents the resist layer from giving off gases (Patent documents 1 and 2). Further, researches have been made on polymers that absorb deep UV light enough to be usable, for the topcoat layer (Patent document 3). Those polymers have benzene, naphthalene or anthracene skeletons so as to improve the effect of the topcoat layer on absorbing deep UV light. For the purpose of further enhancing the absorption of deep UV light, studies have been made on searching for suitable polymers and/or favorable combinations of polymers.